Choose the correct answer from the options given below:

• (1) (A)-(I), (B)-(II), (C)-(III), (D)-(IV)
• (2) (A)-(IV), (B)-(III), (C)-(I), (D)-(II)
• (3) (A)-(I), (B)-(II), (C)-(III), (D)-(IV)
• (4) (A)-(IV), (B)-(III), (C)-(II), (D)-(I)

  The correct answer is option (4): (A)-(III), (B)-(II), (C)-(IV), (D)-(I).

  This matching question tests standard biochemical classification of amino acids by side chain properties, crucial for GATE Life Sciences protein structure topics.

  Matching Explanations

  (A) Negatively charged amino acid → (III) Aspartate

  Aspartate (Asp, D) has -COOH in its side chain that deprotonates at physiological pH (~7), giving net negative charge.

  (B) Polar uncharged amino acid → (II) Cysteine

  Cysteine (Cys, C) features -SH (thiol) group that’s polar due to electronegative sulfur but remains uncharged (pKa ~8.3).

  (C) Aromatic amino acid → (IV) Phenylalanine

  Phenylalanine (Phe, F) contains benzene ring in side chain, defining aromatic amino acids alongside Tyr, Trp.

  (D) Positively charged amino acid → (I) Histidine

  Histidine (His, H) has imidazole ring (pKa ~6.0) often protonated at pH 7, giving positive charge; classic basic amino acid.

  Option Analysis

  Option	A-III	B-II	C-IV	D-I	Correct?
  (1)	✗	✓	✗	✗	No
  (2)	✗	✗	✗	✗	No
  (3)	✗	✓	✗	✗	No
  (4)	✓	✓	✓	✓	Yes

  Only option (4) correctly classifies all by R-group chemistry at physiological pH.

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  Amino Acid Classification by Charge and Polarity for GATE

  Negatively charged polar uncharged aromatic positively charged amino acid matching tests core biochemistry knowledge for protein folding, enzyme active sites in competitive exams. Master Aspartate-Cysteine-Phenylalanine-Histidine linkages for Q.74.

  Negatively Charged: Aspartate Characteristics

  Aspartate’s β-carboxylic acid (pKa ~3.9) deprotonates, contributing negative charge to proteins.

  Polar Uncharged: Cysteine Thiol Properties

  Cysteine’s sulfhydryl enables disulfide bridges, polar but neutral at pH 7.

  Aromatic Amino Acids: Phenylalanine Structure

  Phenylalanine’s hydrophobic benzene ring absorbs UV at 260nm, key for quantification.

  Positively Charged: Histidine Imidazole Function

  Histidine buffers near pH 7, essential in hemoglobin, catalytic triad of enzymes.

  Exam Memory Aid

  Mnemonic: Down (Aspartate-negative), Chilly (Cysteine-polar), Phe-fancy (Phenylalanine-aromatic), His-high (Histidine-positive).